1. Field of the Invention
The invention relates generally to navigation equipment and more specifically to global positioning system receivers which report their self-positioning to other monitors and rescue networks that include such receivers.
2. Description of the Prior Art
A satellite positioning system (SPS) is a system of orbiting satellite signal transmitters, with receivers located on or near the earth's surface, that transmits information from which an observer's location and/or the time of observation can be determined. Two such operational systems are the global positioning system and the global orbiting navigational system.
The global positioning system (GPS) is part of a satellite-based navigation system developed by the United States Defense Department under its NAVISTAR satellite program. A fully operational GPS includes up to twenty-four satellites approximately uniformly dispersed around six circular orbits with four satellites in each orbit. The orbits are inclined at an angle of 55.degree. relative to the equator, and are separated from each other by multiples of 60.degree. longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geo-synchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the earth below.
Theoretically, three or more GPS satellites will be visible from most points on the earth's surface, and visual access to three or more such satellites can be used to determine an observer's position anywhere on the earth's surface, twenty-four hours per day. Each satellite carries a cesium and rubidium atomic clock to provide timing information for the signals transmitted by the satellites. Internal clock correction is provided for each satellite clock.
Four satellites, at a minimum, are needed to uniquely determine x, y, z earth-position and time. If only three satellites are visible, conventional GPS software solves for x, y and time. Time is nearly always needed to be ascertained, and the z-dimension can be constrained, e.g., assumed.
Each GPS satellite transmits two spread spectrum, L-band carrier signals. An "L1" signal has a frequency f1=1575.42 MHz, and an "L2" signal has a frequency f2=1227.6 MHz. These two frequencies are integral multiples f1=(1500)(f0) and f2=(1200)(f0) of a base frequency f0=1.023 MHz. The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, and carries a coarse grained acquisition code (C/A-code) and/or a precision, fine-grained code (P-code). The L2 signal from each satellite is BPSK modulated by only the P-code.
The use of two carrier signals L1 and L2 permits the computation for partial compensation of the propagation delays of the signals through the ionosphere. This ionospheric delay varies approximately as the inverse square of signal frequency f (delay .alpha.f.sup.-2). This phenomenon is discussed by MacDoran in U.S. Pat. No. 4,463,357.
Use of the PRN codes in a code multiple access scheme allows the sorting out of the GPS satellite signals which all share the same L1 and L2 frequencies. A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the corresponding, unique PRN code for that particular satellite. The PRN codes come from a short list, and each is stored in GPS receivers carried by ground observers.
The P-code, is a relatively long, fine-grained code having an associated clock or chip rate of (10)(f0)=10.23 MHz. The C/A-code allows rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser grained code having a clock or chip rate of f0=1.023 MHz. The C/A-code for any GPS satellite has a length of 1023 chips and thus repeats every millisecond. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats. Acceptable methods for generating the C/A-code and P-code are set forth in the document GPS interface control document ICD-GPS-200, published by Rockwell International Corporation, satellite systems division, Revision A, 26 Sep. 1984.
The GPS system is such that the C/A-code and P-code can be deliberately corrupted by random dithering, which inhibits position-fix accuracy. This mode is called selective availability (SA), and includes the transmission of an encrypted Y-code on frequency L2. "Authorized" receivers are required to decode the Y-code and such receivers will retain their accuracy in position fix determination during SA.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite and an almanac for all GPS satellites, with additional parameters providing corrections for ionospheric signal propagation delays suitable for single frequency receivers and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of fifty Baud. A useful discussion of the GPS and techniques for obtaining position information from the satellite signals is found in Guide To GPS Positioning, edited by David Wells, Canadian GPS Associates, 1986.
A second configuration for global positioning is the global orbiting Navigation satellite system (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS also uses twenty-four satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8.degree. relative to the equator, and the three orbital planes are separated from each other by multiples of 120.degree. longitude. The GLONASS circular orbits have smaller radii, about 25,510 kilometers, and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete seventeen and sixteen revolutions, respectively, around the earth every eight days. The GLONASS system uses two carrier signals "L1" and "L2" with frequencies of f1=(1.602+9 k/16) GHz and f2=(1.246+7 k/16) GHz, where k(=0,1,2, . . . , 23) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617GHz (L1) and 1.240-1.260 GHz (L2). The L1 code is modulated by a C/A-code (chip rate=0.511 MHz) and by a P-code (chip rate=5.11 MHz). The L2 code is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at rate of fifty Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code code is the same, for each satellite. The methods for receiving and analyzing the GLONASS signals are similar to the methods used for the GPS signals.
Single frequency (L1 only) and dual-frequency (L1 and L2) GPS receivers are now commercially available that are unauthorized and therefore unable to decode the encrypted Y-code. Dual frequency GPS receivers are capable of receiving more than just the L2 carrier transmissions from GPS satellites. A single frequency GPS receiver system could be associated with L2 transmitters that would cause little, if any, interference on the L1 frequency. Since much or all of the hardware already exists in portable GPS receivers to receive position reports over an otherwise under-used L2 carrier frequency, only software changes to the programming of a conventional GPS receiver may be needed to implement a ground network that has its members' individual positions communicated amongst them.
Conventional GPS receivers can provide downed pilots, for example, with their location, but it may not always be possible for that information to be relayed by the pilot verbally. Ad hoc position reporting can also be difficult, so an automatic means is desirable that has a minimal impact on the kind of hardware required to mount a rescue operation, or to coordinate dispatching mobile units in the field.